1. Field of the Invention
Aspects of the present invention relate to a desulfurization adsorbent for a fuel cell and a desulfurizing method using the same, and more particularly, to a desulfurization adsorbent having an excellent performance for adsorbing sulfur compounds as well as excellent regeneration properties, and a desulfurizing method using the same.
2. Description of the Related Art
A fuel cell is an electricity generating system in which the chemical energy of oxygen and hydrogen contained in hydrocarbon materials such as methanol, ethanol and natural gas is directly converted to electrical energy.
A fuel cell typically includes a stack, a fuel processor (FP), a fuel tank, a fuel pump, etc. The stack constitutes the main body of the fuel cell and has a structure in which a few to a few tens of unit cells are laminated, with each unit cell consisting of a membrane-electrode assembly (MEA) and a separator (or bipolar plate). The fuel pump supplies fuel from the fuel tank to the fuel processor, and the fuel processor reforms and purifies the fuel to generate hydrogen, which is fed to the stack. The hydrogen reaching the stack electrochemically reacts with oxygen to generate electric energy.
A reformer and a water-gas shift reactor in the fuel processor employ a reforming catalyst and a shift catalyst, respectively, to reform a hydrocarbon fuel and to remove carbon monoxide. Typically, hydrocarbon as a raw fuel for production of hydrogen contains sulfur compounds, which is undesirable since the catalysts of the reformer and water-gas shift reactor and the anode catalyst of the membrane-electrode assembly are susceptible to poisoning by sulfur compounds. Therefore, it is necessary to remove sulfur compounds before supplying the hydrocarbon to a reforming process. Accordingly, the hydrocarbon is subjected to a desulfurization process prior to entering the reforming process (See FIG. 1).
In particular, there is a high possibility in the future that city gas (such as, for example, gas supplied by a municipal utility) will be used as the feedstock for fuel cells. However, city gas contains about 15 ppm of a mixture of sulfur compounds that act as odorizing agents, namely, TBM tertiary butyl mercaptan (TBM) and tetrahydrothiophene (THT) at a ratio of 3:7, and it is essential to remove these sulfur compounds from city gas before using city gas in fuel cells.
In order to remove sulfur compounds, a hydrodesulfurization (HDS) process may be used, or a method of using an adsorbent may be used. The hydrodesulfurization process is a reliable process, but requires high temperatures such as 300 to 400° C. and involves complicated operations. Thus, the hydrodesulfurization process is more suited to large-scale plants than pilot scale apparatuses.
Therefore, for smaller-scale apparatuses, it is more appropriate to use an adsorbent for the removal of sulfur compounds such as TBM and THT from a fuel gas. The method of using an adsorbent includes passing the fuel gas through an adsorbent bed, which typically is made of activated carbon, metal oxide or zeolite, to remove sulfur compounds. When the adsorbent becomes saturated with the sulfur compounds, the absorbent becomes unable to remove sulfur compounds from the fuel gas. At that time, the adsorbent needs to be replaced or regenerated. The amount of the adsorbent required and the replacement period for the adsorbent largely depend on the adsorptivity of the adsorbent, and thus an adsorbent having high adsorptivity is advantageous.
Various adsorbents have been suggested. For example, Japanese Patent Application Laid-Open No. Hei 6-306377 discloses a zeolite that is ion-exchanged with multivalent metal ions and that removes mercaptans from city gas. However, this zeolite is unfortunately applicable only to mercaptans.
Among the sulfur compounds mentioned above, THT is more difficult to remove than TBM. It is known that a zeolite containing silver (Ag) has an ability to remove THT. Japanese Patent Application Laid-Open No. Hei 10-237473 describes an adsorbent comprising a Na—X zeolite having a pore size of at least 5 Å. This adsorbent exhibits excellent adsorptivity at ambient temperature, but the adsorptivity drastically decreases when the adsorbent is exposed to moisture.
However, among the desulfurization adsorbents disclosed so far, there has been no desulfurization adsorbent which has excellent adsorption performance as well as regeneration properties, and hence, there is still a demand for improvements in the performance of conventional desulfurization adsorbents.